The Annual Distribution of the Crustacea. 809 





A 



on their development. At the time of the maximum Cyclops 

 may number more than 2,500,000 per sq. m. of surface, but of 

 this enormous number only a very small fraction ever become 

 sexually mature. In any catch made at this season of the year, 

 not more than five per cent, are mature, and not more than one 

 or two per cent, are egg-bearing females. The great majority, 

 therefore, of these Cyclops die without reaching maturity, and 

 after the maximum has been passed the number of Cyclops de- 

 creases even more rapidly than it rose. The decline may go so 

 far that in June the number of this species is scarcely larger 

 than in March. 



During this decHne of Cyclops, the other perennial species are 

 increasing in number, but their combined increase is more 

 than counterbalanced by the decrease in the number of Cyclops, 

 so that the late spring and early summer show a marked decline 

 in the total number of Crustacea. 



m 





The Crustacea in Summer. 



The summer life of the Crustacea begins with the decline from 

 the spring maximum to the early-summer minimum. This de- 

 cline is dependent in part on the decrease of Cyclops. In part, 

 also, it depends on the fact that both species of Daphnia regu- 

 larly decline after a brief maximum in late May or early June, 

 and in 1895 Diaptomus showed the same decline. The total 

 number of Crustacea may be thus reduced to one-fourth, or less, 

 of the number present at the spring maximum. The lowest 

 point of numbers was about the middle of June in 1896, and 

 about the first of July in 1895. In 1894, when observations be- 

 gan, during the first week of July, the Crustacea were apparently 

 at their minimum, which was exceptionally low in that year, 

 owing to the peculiar character of the vegetation during that 

 season. It was not greater than the number in the winter of 

 1895-96. 



The Crustacea increase in number after the early-summer 

 minimum. This increase seems to be due to two causes. First, 

 the development of species hitherto represented in small num- 

 bers. In all years there comes at this time an increase of Cy- 

 clops Leuckartii. The numbers of this species differ greatly in 



